Electrodeposition of an aluminumcontaining coating



United States Patent 2,911,341 ELEcTRon PosrrIoNoF AN LUMINUM- CONTAINING COATING Robert F. Linden, Chicago, Ill., assignor to trustees for the stockholders of the Major Engineerin Corporation, I

Chicago, 111., a corporation of Illinois No Drawing. Application May 2 1957 Serial No. 656,498

tion Serial No. 457,770, filed September 22, 1954, now abandoned.

The present invention relates to a process for the deposition of metallic aluminum coatings upon metallic surfaces, to the electrolyte used in this process and to the articles produced by this process. The process concerns itself primarily with the electro-deposition of aluminic coatings on ferrous metals and alloys, more especially upon iron sheets.

It has been considered impossible, up to the present time, to produce adherent and permanent aluminic de posits upon iron surfaces 'by processes involving electrodeposition. While it was well known, and widely practiced, to produce aluminum coatings upon iron by hotdipping methods, such as by the process known as calorizing, this is a rather expensive proceeding and entails the use of relatively high temperatures, which adversely affect the characteristics of the iron or steel objects which are subjected to the treatment.

While the electrolytic decomposition. of aluminum salts into free metallic aluminum forms the basis of the commercial production of aluminum, this process requires a highly heated bath of aluminum oxide dissolved in molten cryolite. the production of relatively thin but tightly adherent aluminum-containings upon iron surfaces.

Accordingly it is one of the objects of the present invention to provide a method for the production of iron articles, such as sheets, by electroplating them with aluminum or aluminic compounds deposited from an aqueous solution of an aluminum salt.

A further object of the invention is to provide iron objects having an adherent electro-deposited coating of an aluminum-containing coating. j

A still further object of the present invention is to provide a special type of aqueous electrolyte from which metallic aluminum may be eletcroplated upon an iron surface in an inexpensive and facile manner.

Ordinarily, when a unidirectional electric current is passed through a solution of an electrolyte, the cations therein, such as hydrogen or a metal, are attracted to the cathode, while oxygen, or the ion of' the particular salt, is attracted to the anode. The current of electricity is carried through the solution from the anode to the cathode. When the conditions are such that the cathodic deposition "of the metal is faster than the deposition of the hydrogen, the metal will appear as a more or less coherent deposit upon the cathode. Whether the coating will be a loose one, or an adherent one, depends upon many factors, such as concentration and nature of the electrolyte, current density, voltage, amperage, temperature, and in some cases, the presence of certain colloidal materials.

By reason of its ready solubility in alkaline solutions, it had been deemed impossible to vdeposit an aluminic coating upon a cathode, as the rate of deposition of the.

coating usually was less than the rate of dissolution of Such a process is entirely unsuited for 2,911,341 Patented Nov. 3, 1 959 2. the coating from the cathode by the alkaline material formed at the cathode; which is particularly true where the conditions are such that sodium hydroxide will form at the cathode where the hydrogen is likewise being set free.

For this reason, the electrodeposition of metallic aluminum or aluminic compounds has been deemed impossible. a

In accordance with the presence invention, however, it has been found that conditions may be so adjusted that metallic aluminum or aluminic compounds will nevertheless become deposited upon the cathode, particularly if the latter be a clean iron surface It was foundthat what was primarily required was the presence of an excess of the nonmetalli'c ions of the particular aluminum salt which was beingv used as therelectrolyte.

As the present invention lends itself very admirably to the production of aluminum-coated iron sheets, such as may be used for sheetiron construction, manufacture V of cans, and the like, it. will be described in connection with the complete treatment of such iron sheets. This, however, is to be understood as in no way intending to limit the present invention in any way, as it is self-evident that it may be used for the coating of any metallic object, particularly those having an iron or steel surface. Where, hereinafter, the term iron is used, it should be understood that this will include iron alloys such as steel.

It was found that when using aqueous aluminum sulfate as an electrolyte a quantity of some other soluble sulfate salt had to be present in solution in order to yield satisfactory results. The amount of these other soluble sulfate salts required to be present in the aluminum sulfate electrolyte is dependent upon the amount of sulfate ions carried by the other sulfate salts. For the proper operation of the electrolyte the amount of sulfate ions provided by the other soluble sulfate salts' should be at least 10% as much as the sulfate ions forming part of the aluminum sulfate which is the primary eletcrolyte constituent. For example, if the electrolyte contains 1000 gramsof A1280; 18H O it isevident that 344 grams ofsulfate ions (SO will be supplied to the solution. Thus at' least 34.4 grams (10% o'f'34'4 grams) of sulfate ions must be supplied by another soluble salt. If, again forexample, this other salt is' Na SO -10H O it can be seen by simple calculation that at leastgrams, of this salt must be added to the electrolyte.

The excess sulfate ionsfmay be supplied to the electrolyte by the sulfates of the alkali metals sodium and potassium, the sulfate of thealkaline earth metal magnesium and by ammonium sulfate. Although the required excess sulfate ions may be contributed by any one or combination of these soluble sulfates, it is preferable to provide a mixture thereof inasmuch as several of the sulfates appear .to contribute different beneficial properties. to the electrolyte. Specifically ithas been found that sodium sulfate and ammoniumsulfate tend to increase the electrical conductivity of-the electrolyte; magnesium sulfate appears to promote the electrodeposition of the aluminum, and sodium thiosulfate appears to be an excellent reducing compound effective for removingoxygen from the electrolyte.

More specifically, it is preferred that the aqueous electrolyte solution be operated in the following range of composition:

Grams per liter of :the plating is adversely affected, whereas above 4.5

hydroxides of the various constituent cations begin to form and precipitate. As the plating proceeds the electrolyte Wlll tend to become more alkaline because apparently the system has a greater cathode efiiciency than anode efficiency. The pH of the solution is maintained within the desired range by additions of sulfuric acid.

It has been found that the current density should be maintained above 16 amperes per square foot although it is preferred to operate within the range of 30 to 4-0 amperes per square foot as better results are achieved in this range.

The solution can be quite satisfactorily operated at room temperature. As a practical matter, the solution must be maintained during plating in a liquid state below 140 F. At temperatures higher than 140 F. basic hydroxide compounds of aluminum and magnesium will tend to precipitate from the electrolyte solution unless, of course,

. the pH range is altered toward the more acidic.

The surface to be plated is made cathodic, and pure aluminum anodes are used. It has been found that very superior results can be obtained if the surface to be plated, i.e. the cathode, is of a ferrous metal or alloy. The use of pure aluminum anodes is possible because with the electrolyte of this invention anodic oxidation is substantially retarded.

In-operation the surface to be coated, which is preferably a ferrous surface, is thoroughly cleaned by means of 7 an alkaline solution. This may be accomplished by making the ferrous article orsurface the anode, while immersed in an aqueous solution containing 4 ounces avoirdupois of sodium carbonate and 2 ounces avoirdupois of sodium phosphate per gallon of water, and allowing the solution, maintained at a temperature of about 176 F., to act upon the surface of the article for about five minutes at about 10 amperes. The article may then be withdrawn and washed with hot water. It may then be made anodic and pickled in an acid solution containing about 6 ounces avoirdupois of concentrated hydrochloric acid per gallon of water and an amount ofaluminum chloride up to the saturation point of the liquid for that salt. This treatment is effected for about 1 minute at about 6 volts. The ferrous surface by that time should be completely clean of all scale, rust and adhering material, and the article may then be withdrawn and washed with water to remove the acid solution. It is then ready for the electroplating operation.

The electroplating is accomplished using an aqueous electrolyte previously described, the main ingredient of which is aluminum sulfate, and which contains quantities of sodium sulfate, ammonium sulfate, magnesium sulfate and sodium thiosulfate. This electrolyte should be maintained within the pH range of 3.5 to 4.5, and it is preferred that the temperature of the electrolyte be between 70 and 90 F.

To effect the electrodeposition of the desired aluminic layer, the cleaned iron surface or article is connected to the negative side of a source of unidirectional electric current, i.e. it is made the cathode, while there is also inserted into the electrolyte an aluminum anode, which may be relatively pure aluminum, say 99% or better. There is then applied a current of about 1 to 6 volts, preferably about 2 /2 volts, at about 6 amperes, the distance between the cathode and anode being about three inches. The current density is preferably about 30 amperes per square foot of iron surface. If both sides of an iron sheet are to be plated, it will be necessary to use an aluminum anode on each side of the sheet.- The sheet may be stationary, but it is to be considered as within" the scope of the present invention to keep it in motion, as, for instance,

by passing a long sheet of iron or steel-through the electroplating bath, as is commonly done when electrogalva' nizing or electrotinning. It is advantageous to keep the electrolyte in a good state of agitation; but this is not absolutely essential, but will give better and more even deposits of aluminum. In a still tank, and without agitation, the time of treatment may vary from five minutes up to one hour.

It will be found that the thickness of the deposited coating bears no definite relationship to the time of treatment nor to the strength or amount of electric current used, as it appears that when once a complete coverage of the iron surface has been attained, no further aluminum will be deposited. Extensive tests have shown that the thickness of the deposited aluminic coating averages from about 0.000079 to 0.00009l inch; and that the weight of aluminic deposit per square inch of iron surface averages from about 00035 to about 0.00405 gram. It also appears that the aluminiferous layer itself carries an ex tremely thin but integral coating of aluminum oxide, as is common with most aluminum surfaces.

After the electroplating is completed, the article is thoroughly rinsed with hot water and allowed to dry.

It will be found that the articles thus treated are evenly and thoroughly coated with aluminum, and that they will be highly resistant to rust and corrosion. They will withstand the usual atmospheric exposure tests, salt spray tests, boiling in water, immersion in Water over long periods of time, and similar tests which are usually employed for testing tinplate for its suitability. Cans made from steel sheets coated in the manner hereinabove described have been made up and it was found that the food packed therein remained unspoiled for a long time. Experimentally, and to test out the efficacy of the present invention, cans with food in them have been kept for over three years without deterioration either inside or outside.

Iron or steel articles coated in accordance with the teachings of the present invention may be used in place of galvanized, tin-coated, or cadmium plated articles with substantially the same results. The process affords an inexpensive and very efficacious means for rustproofing iron. Moreover, the aluminic surface obtained, mainly by reason of the presence of the additional film of aluminum oxide, lends itself to painting, 'lithographing and other forms of applied coating operations. As aluminum is an inexpensive material, the cost of providing electrodeposited aluminic coatings is quite low, and the process is therefore competitive with tinplating and galvanizing.

The articles present a gray, dull surface, but this can readily be buffed to a state of high luster, if desired. The surface also lends itself to the well known anodizing operation whereby to produce thereon a lustrous bright finish.

The invention is not limited to any particular type of apparatus or source of direct current. Thus it might be supplied by a suitable dynamo, or by rectification of alternating current. Means for controlling voltage, amperage, and current density, and apparatus for handling iron or steel sheets or articles are so well known that they require no illustration.

Although I have described my invention with a certain degree of particularity it is understood that the'present disclosure has been made only by way of example and that numerous changes in the electrolyte composition and in the plating procedure may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed.

I claim:

1. The process of electroplating an aluminum'-containing coating onto a metallic surface which comprises making the surface the cathode and passing a unidirectional current through an aqueous solution containing at least 15 grams per liter of aluminum sulfate and at least one i salt' selected from the group consisting of the alkali metal excess of sulfate ions over that contributed by the aluminum sulfate, with an essentially pure aluminum anode and maintaining the solution within the pH range of 3.5 to 4.5.

2. The process of electroplating iron with an alumi hum-containing coating which comprises passing a unidirectional electric current through a nonanodizing aqueous solution containing at least 15 grams per liter of aluminum sulfate and sufficient quantities of at least one soluble salt selected from the group consisting of the alkali metal sulfates, the alkaline earth metal sulfates and ammonium sulfate to provide at least a excess of sulfate ions over that contributed by the aluminum sulfate, with the iron being the cathode, and with an essentially pure aluminum anode and maintaining the solution within the pH range 3.5 to 4.5.

3. The process of electroplating iron with an aluminum-containing coating which comprises passing a unidirectional electric current through an aqueous solution containing at least grams per liter of aluminum sulfate, and such quantities of magnesium sulfate, sodium thiosulfate, ammonium sulfate, and a sulfate of at least one element selected from the group consisting of sodium and potassium to provide at least a 10% excess of sulfate ions over that contributed by the aluminum sulfate, with the iron being the cathode and with an essentially pure aluminum anode and maintaining the solution within the pH range of 3.5 to 4.5.

4. The process of electroplating an aluminum-containing coating from an essentially pure aluminum anode onto a ferrous surface which comprises passing a unidirectional current through an aqueous solution containing at least 15 grams per liter of aluminum sulfate and at least one soluble sulfate salt selected from the group consisting of sodium sulfate, sodium thiosulfate, potassium sulfate, ammonium sulfate and magnesium sulfate in sufficient quantity to provide at least a 10% excess of sulfate ions over that contributed by the aluminum sulfate with the ferrous surface being the cathode and maintaining the solution within the pH range 3.5 to 4.5.

5. The process of electroplating iron with an aluminum-containing coating which comprises passing a unidirectional electric current at a current density above 15 amperes per square foot through an aqueous solution containing at least 15 grams per liter of aluminum sulfate and at least one soluble salt selected from the group consisting of the alkali metal sulfates, the alkaline earth metal sulfates and ammonium sulfate in sufficient quantity to provide at least a 10% excess of sulfate ions over that contributed by the aluminum sulfate, making the iron the cathode and a mass of aluminum the anode and maintaining the electrolyte within the pH range of 3.5 to 4.5.

6. The process of electroplating iron with an aluminum-containing coating which comprises making the iron the cathode and passing a unidirectional electric current at a current density of at least 15 amperes per square foot through an aqueous electrolyte solution containing at least 15 grams per liter of aluminum sulfate, an alkali metal sulfate, ammonium sulfate and magnesium sulfate in sufficient quantity to provide at least a 10% excess of sulfate ions over that contributed by the aluminum sulfate with an essentially pure aluminum anode and maintaining the solution within the pH range of 3.5 to 4.5.

7. The process of electroplating an aluminum-containing coating from an essentially pure aluminum anode onto a ferrous surface which comprises passing a unidirectional electric current at a current density of above 15 amperes per square foot through an aqueous electrolyte solution containing at least 15 grams per liter of aluminum sulfate and a soluble sulfate selected from the group consisting of sodium sulfate, sodium thiosulfate, potassium sulfate, ammonium sulfate and magnesium sulfate, in suflicient quantity to provide at least a 10% excess of sulfate ions over that contributed by the aluminum sulfate, making the ferrous surface the cathode and a mass of aluminum the anode, and maintaining the solution within the pH range of 3.5 to 4.5.

8. The process of electroplating iron with an aluminum-containing coating which comprises passing a unidirectional electric current at a current density of between 30 to 4 0 amperes per square foot through an electrolyte consisting of an aqueous solution of the following ingredients in the amounts indicated:

Grams per liter A1 so, ,-18H,o 9010120 N212SO4-10H2O 30m 45 (NH4)2SO4 7.5 to MgSO '7H O 30120 45 Na-2S2035H2O 7.51:0

Al (SO 'l8H O 90110 Na SO l0H O 3 0 to (NH4)2SO4 7-5 to MgSO 71-1 0 30 to 45 Na S O 7.5 to

said solution having a pH within the range of 3.5 to 4.5.

References Cited in the file of this patent UNITED STATES PATENTS 436,895 Jeancon Sept. 23, 1890 2,361,378 Brennan Oct. 31, 1944 FOREIGN PATENTS Great Britain Dec. 19, 1885 

1. THE PROCESS OF ELECTROPLATING AN ALUMINUM-CONTAINING COATING ONTO A METALLIC SURFACE WHICH COMPRISES MAKING THE SURFACE THE CATHODE AND PASSING A UNDIRECTIONAL CURRENT THROUGH AN AQUEOUS SOLUTION CONTAINING AT LEAST 15 GRAMS PER LITER OF ALUMINUM SULFATE AND AT LEAST ONE SALT SELECTED FROM THE GROUP CONSISTING OF THE ALKALI METAL SULFATES, THE ALKALINE EARTH METAL SULFATES AND AMMONIUM SULFATE IN SUFFICIENT QUANTITY TO PROVIDE AT LEAST A 10% EXCESS OF SULFATE IONS OVER THE CONTRIBUTED BY THE ALUMINUM SULFATE, WITH AN ESSENTIALLY PURE ALUMINUM ANODE AND MAINTAINING THE SOLUTION WITHIN THE PH RANGE OF 3.5 TO 4.5. 